The invention relates to the production of lost cores or moulded parts for casting moulds in order to produce cast components (cast part production). Lost cores are required in the casting industry for the production of voids inside cast parts and may have an extremely complex structure, for example cores for vehicle cylinder heads.
The lost cores are produced from a grainy and dry basic moulding material, a sand, usually quartz sand, but also from chromite, zirconia, olivine, feldspar, mullite or other sands and a chemically hardening binder system. These components are mixed, if necessary with the addition of further additives and by applying pressure (compressed air) to the moulding die (core or mould box). Subsequent solidification of the moulding mixture, which is still loosely packed, can be carried out by various methods, for example by the passage of a hardening gas, for example carbon dioxide, or a thermal solidification using a heated, i.e. metallic moulding die.
Binders which are based on water glass are known which usually consist of multi-component systems formed by the water glass component and an additive component which is usually in powder form. Mixed with basic moulding materials and moulded into cores or moulded core parts, moulded parts of this type can be solidified by physical solidification (dehydration, removal of water, drying) or chemical solidification (chemical hardening).
The known INOTEC process uses a binder mixture based on sodium silicate as the binder, whereby the properties of this moulding material are improved by adding additives, known as INOTEC promoters. Hardening in this case is carried out by means of a stepwise dehydration of the core moulding material using die temperatures between 150° C. and 250° C. as well as subsequently flushing with hot air in the same temperature range. Depending on the quality of the sand, binder contents of between 1.8% and 2.5% and promoter contents of between 0.1% and 1.0% are used. The promoter improves, inter alia, the flowability and strength of the core moulding material and the core by binding individual binder particles together and forming a three-dimensional network.
The main areas of application of the INOTEC process and the INOTEC binder are light metal and non-ferrous metal alloys, for example for the production of cylinder heads, and also for valve body casting.
In addition, a CORDIS binder system is known, in which the binder comprises a matrix formed from a combination of phosphate, borate and silicate groups. The CORDIS binder system is an inorganic two-component system which is composed of the CORDIS binder and the additive Anorgit. To use this binder system, a heated core box (130° C. to 180° C.) as well as a hot air flush (100° C. to 200° C.) are necessary. The binder quantities employed can be between 1.5% and 3.0%, depending on the type of binder. In this manner, bending strengths of 350 to 550 N/cm2 are obtained.
The CORDIS binder system is also used for the production of inorganic bonded cores in temperature-controlled core production dies. The CORDIS binder system is composed of water as the solvent and an inorganic binder matrix. Depending on the application, this binder matrix consists of a combination of modified phosphate, silicate and borate groups. Furthermore, by adding inorganic substances directly to the binder or by means of an additive during production of the core, the properties can be specifically altered. Examples of these include flowability, reactivity of the moulding mixture, wetting of the core by the melt or the shelf life.
The problem with this binder is that the shelf life of bonded cores is limited because of its hydrophilic nature. As an example, when stored for 24 hours, the strength drops by approximately ⅓ of the starting strength. Preferably, the CORDIS binder systems are used for aluminium gravity shell casting.
A further alternative for the emission-free production of inorganic cores is what is known as the AWB (inorganic hot box) process. The AWB process is also operated with heated core boxes (160° C. to 200° C.). When the core is shot, a vacuum is applied to the core box which acts to remove the steam which is formed. Hardening is purely physical in nature, which has a positive influence on regeneration of the moulding material following casting. After the core in the core box has reached a certain processing strength, it is completely hardened in a microwave oven at low power.
The AWB process is based on thermal hardening of water glass-bonded moulding materials in a temperature-controlled die with subsequent microwave drying. The binder is a modified water glass which has a low viscosity by being diluted with sodium hydroxide. The flowability of the moulding material mixtures produced in this manner and thus their shot capabilities leads to good production of the core or moulded part. Solidification of the moulding material in the AWB process is exclusively obtained via dehydration, i.e. drying at die temperatures between 160° C. and 200° C., wherein in addition, a vacuum can be applied. The subsequent drying is then carried out using low power microwave ovens. The binder contents are between 1.5% and 2.5%. No additives are used.
DE 103 21 106 A1 discloses a moulding material for moulded parts from casting moulds for casting light metal melts, wherein the basic moulding material used is a quartz-free sand (olivine) and an inorganic binder based on water glass.
Powdered water glass systems are also known in which spray dried water glass is used as the binder. However, the disadvantage here is that this powdered binder is constituted by microfine particles which pollute the air at the foundry site.
Two-component binder systems are known in the prior art. DE 20 2008 017 975 U1 discloses a two-component system formed by a first liquid, water glass and a second solid, a component containing a particulate metallic oxide. In addition, a surfactant is added, preferably to the liquid component. The particle size of the metallic oxide here is less than 100 μm and more than 10 μm. The disadvantage here is that this binder requires the addition of a surfactant. In addition, the binder is prepared as a two-component system and must firstly be mixed together in a complicated manoeuvre prior to use.
DE 2434431 A1 discloses a binder system based on water glass, wherein the mixtures of moulding material formed contain a series of additional components in addition to the basic moulding material and the binder. The binder has a ratio of silica to alkaline oxide of between 3.5:1 and 10:1 and is added to the basic moulding material in proportions of between 3% and 15% by weight. The additives which are used for this multi-component system are clay or alumina, carbonaceous materials (for example pitch or carbon black) as well as film-forming resin binders (for example polyvinylacetate dispersions or vinyl acetate-ethylene copolymers).
DE 10 2012 020 510 A1 discloses a mixture of moulding materials formed from a refractory basic moulding material, an inorganic binder based on water glass as well as particulate amorphous SiO2. It also contains additional organic additives as well as various surfactants with a system that functions as a two-component binder. The particulate amorphous SiO2 is added as a powder in this instance. The moulding material mixture contains a hardener (for example an ester or phosphate compound) and is suitable for use in aluminium casting. The moulding material mixture is hardened with the aid of hot dies which are preferably heated to 120° C. to 250° C.
DE 10 2007 027 577 A1 discloses a moulding material mixture which, in addition to the binder based on an alkali silicate, contains 0.1% to 10% of sodium hydroxide as well as an addition of between 0.1% and 3% by weight of a suspension with a solids content of between 30% and 70% of amorphous SiO2 beads. Microwave energy is used to dry the moulding material mixtures that are produced thereby.
CN 1721103 A discloses an inorganic binder for moulded part production with an improved disintegration behaviour after casting the cast parts. The binder contains dextrose powder, calcium carbonate powder, a suspension agent and further additives.
DE 10 2007 023 883 A1 discloses a shot gas supply device for supplying a core shooter with a moist gas with a specific moisture content, wherein the temperature of this gas can also be varied. A microwave emitter can be used to control the temperature of the moist gas.
EP 2 163 328 A1 discloses a process in which the basic moulding material is coated with between 0.25% and 0.9% with respect to the total weight of the moulding material of a water glass binder; the binder additionally contains at least one additive from the group formed by bonding agents, flow improvers, surface improving agents, drying agents or separating agents. The moulding material mixture also contains at least one hardener which, for example, hardens in contact with steam. Hardening of the moulded parts produced is carried out in a heated moulding die which is preferably heated to temperatures between 60° C. and 120° C.
EP 1 095 719 A2 discloses a binder system for moulding sands for the production of cores. The binder system based on water glass consists of an aqueous alkali silicate solution and a hygroscopic base such as sodium hydroxide, for example, which is used in a ratio of 1:4 to 1:6. The water glass has a SiO2/M2O modulus of 2.5 to 3.5 and a solids content of 20% to 40%. In order to obtain a moulding mixture which can be poured and which can also be used to fill complicated core moulds, and also to control the hygroscopic properties, the binder system also contains a surfactant such as silicone oil, which has a boiling point of ≥250° C. The binder system is mixed with a suitable refractory material such as quartz sand, and can then be shot with a core shot machine into the core box. Hardening of the moulding material mixture is carried out by removal of the water it still contains. Drying or hardening of the casting mould may be carried out with the aid of microwaves.
WO 2006/024540 A2 describes that the strength of casting moulds both directly after moulding and hardening and also during storage under increased humidity can be substantially improved by using a binder which contains an alkali water glass as well as a particulate metallic oxide which is selected from the group silicon dioxide, aluminium oxide, titanium oxide and zirconium oxide. The particle size of these metallic oxides is preferably less than 300 μm, particularly preferably less than 100 μm. When producing the moulding material mixture, in general, the refractory basic moulding material is initially provided, and then the binder is added, with stirring. In this regard, the water glass as well as the particulate metallic oxide can be added in any order. However, advantageously, the liquid component is added first. The disadvantage here is that with this moulding material system too, heated moulding dies have to be used.